E594 High

Navien E594 Fault Code

The Navien E594 fault code means: your boiler's computer memory has encountered a serious internal error and can no longer process instructions correctly, meaning the unit cannot operate.. It appears on 7 Navien models in our database.

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description: Abnormal operation: EEPROM (PCB)
Typical repair cost £250-450 (including labour)

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Navien models that show the E594 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Navien E594 fault code information on this page is sourced and cross-checked against the official Navien service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026